The present invention relates to a novel oxazolidine carboxylic acid and a process for its preparation. The invention further relates to a process for the preparaton of paclitaxel (taxol) using such oxazolidine carboxylic acid. Paclitaxel (Taxol) (1) is a terpene of taxane family and has the formula shown below and has an application for treatment of various types of cancer. Non-natural analog of paclitaxel (taxol), docetaxel (taxotere) (2) is also an approved anticancer drug. Thus, there is great interest in molecules having similar structures for development of new anticancer drugs and SAR studies. 
1. Taxol R1=H, R2=Ac, R3=
2. Taxotere R1=R2=H, R3=
3. Baccatin R1=R3=H, R2=Ac
4. 10-Deacetylbacctin (DAB) R1=R2=R3H
The structure of paclitaxel (taxol) has two distinct units. One is baccatin (3) and other is N-benzoylphenylisoserine, the side chain, connected to baccatin at C-13 through ester linkage. It has been shown that the -amido-hydroxy ester side chain at C-13 is very essential for anticancer activity of taxol and any other taxol analogs. The supply of taxol from natural sources is very limited and so there is great interest in its synthesis. 10-deacetyl baccatin (4) is more readily available than taxol from the leaves of yew tree and comprises a starting material for semisynthesis of taxol and taxol analogs. It is known in the literature, that it is very difficult to esterify 13 hydroxy of bacctin with -amido-hydroxy carboxylic acid. The difficulty has been ascribed to spatial disposition of 13 hydroxy in the baccatin nucleus. Therefore, considerable efforts have been made to find precursors of -amido-hydroxy carboxylic acid which can be coupled with bacctin in high yield and the resultant couple product can be processed to afford 13-O-(-amido-hydroxycarbonyl) baccatin in high yield and purity.
The following types of cyclic derivative of phenylisoserine are known to couple with 13-hydroxy of baccatin. 
R, R1, R2 and R3=Protecting groups
In the literature, various protecting groups, R1, are described in oxazolidinecarboxylic acid (5). However, alk-2-ynyloxycarbonyl group have not been described so far; The utility of this group over other commonly used protecting groups lies in the fact that this group can be cleaved under neutral condition. All other known protecting groups (R1) are cleaved under either acidic conditions or by hydrogenolysis. The subsequent cleavage of protecting groups R2 and R3 are normally very fast, once R1 is cleaved. Since the baccatin part of taxol is very prone to degradation even under mild acidic or basic conditions, removal of alk-2-ynyloxycarbonyl group under neutral condition facilitates the conversion of taxol intermediate such as 10 to taxol in high yield and high purity.
An object of this invention is to propose a novel oxazolidine carboxylic acid and a process for the preparation thereof.
Another object of this invention is to propose oxazolidine carboxylic acid which can advantageously be used in the preparation of anticancer drugs.
Still another object of this invention is to propose a process for the preparation of taxol and its synthetic analogs using the proposed oxazolidine carboxylic acid.
The present invention is directed to 3-(alk-2-ynyloxy)carbonyl-5-oxazolidine carboxylic acid and its analogs having a formula 
and wherein R1 is hydrogen, aryl, heteroaryl, alkyl alkenyl, alkynyl, R2 and R3 are independently selected from hydrogen, alkyl, alkenyl, alkynyl, aryl, heteroaryl; R4 is hydrogen, alkyl, alkenyl, alkynyl, aryl, substituted aryl, heteroaryl. R5 and R6 independently selected from hydrogen, alkyl, alkenyl, alkynyl, arly, heteroaryl, alkoxy, alkeyloxy, alkynyloxy, aryloxy, heteroaryloxy.
The oxazoldine alkyl groups either alone or with the variable substituents defined above are preferably lower alkyl containing from one to six carbon atoms in the principal chain and up to 10 carbon atoms. They may be straight or branched chain and include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, hexyl and the like. The alkyl part of alkoxy groups defined above are same as a alkyl groups.
The oxazoldine alkenyl groups either alone or with the various substituents defined above are preferably lower alkenyl containing from two to six carbon atoms in the principal chain and up to 10 carbon atoms. They may be straight chain and include ethenyl, propenyl, isopropenyl butenyl, isobutenyl, pentenyl, hexenyl, and the like. The alkenyl part of alkenyloxy groups defined above are same as a alkenyl groups.
The oxazoldine alknyl groups, either along or with the various substituents defined above are preferably lower alkynyl containing from two to six carbon atoms in the principal chain and up to 10 carbon atoms. They may be straight chain and include ethynyl, propynyl, butynyl, isobutynyl, pentynyl, hexynyl and the like. The alkynyl part of alkynyloxy groups defined above are same alkynyl groups.
The oxazoldine aryl moieties eiher alone or with various substituents contain from 6 to 10 carbon atoms and include pheny, -naphthyl etc. substituents include alkanoxy, hydroxy, halogen, alkyl, aryl, alkenyl, acyl, acyloxy, nitro, amino; amido etc.
As defined herein, the term aryloxy includes aromatic heterocyclic moieties the term aryl includes any compound having an aromatic ring of which no hetero atom is a member, and the term heteroaryl includes any compound having an aromatic ring which comprises a hetero atom.
Most preferably, the oxazolidine carboxylic acid has a formula wherein R1, R2 and R3 are hydrogen, R4 is phenyl and R5 and R6 are methyl. 
The present invention also describes process of preparation of 3-(alk-2-ynyloxy)carbonyl-5-oxazolidine carboxylic acid and its analogs.
The present invention also describes the preparation of taxol intermediates of following structural formula: 
wherein
A and B are independently hydrogen or lower alkanoyloxy, alkenoyloxy, alkynoyloxy aryloyloxy or; A and B together form an oxo;
L and D are independently hydrogen on hydroxy or lower alkanoyloxy, alkenoyloxy, alkynoyloxy, aryloyloxy alkoxycarbonyloxy, aryloxycarbonyloxy or alkylsilyloxy;
E and F are independently hydrogen or lower alkanoyloxy, alkenoyloxy, alkynoyloxy aryloyloxy; or E and F together form an oxo;
G is hydrogen or hydroxy or lower alkanoyloxy, alkenoyloxy, alkynoyloxy, or aryloyloxy or;
G and M together form an oxo or methylene or oxirane ring or
M and F together form an oxetane ring;
J is hydrogen or hydroxy or lower alkanoyloxy, alkenoyloxy, alkynoyloxy, or aryloyloxy or
I is hydrogen or hydroxy or lower alkanoyloxy, alkenoyloxy, alkynoyloxy, or aryloyloxy; or
I and J together form an oxo; and
K is hydrogen or hydroxy, lower alkoxy, alkenoyloxy, alkenoyloxy, alkynoyloxy, or aryloyloxy
P and Q are independently hydrogen or hydroxy, or lower alkanoyloxy, alkenoyloxy, alkynoyloxy, aryloyloxy, alkoxycarbonyloxy, aryloxycarbonyloxy or alkylsilyloxy or P and Q together form an oxo; and
S and T are independently hydrogen, lower alkyl, alkoxy, alkenyl, alkynyl, aryl, aryloxy, substituted aryl or substituted aryloxy; and
U and V are independently hydrogen or lower alkyl, alkenyl, alkynyl, aryl or substituted aryl or heteroaryl and
W and Wxe2x80x2 are independently hydrogen or lower alkyl, aryl or substituted aryl; and
Y is hydrogen or lower alkyl, aryl or substituted aryl.
The present invention also describes the preparation of taxol intermediates, natural taxol and non-natural occurring taxols of following structural formula: 
wherein
A and B are independently hydrogen or lower alkanoyloxy, alkenoyloxy, alkynoyloxy, aryloyloxy, or;
A and B together form an oxo;
L and D are independently hydrogen or hydroxy or lower alkanoyloxy, alkenoyloxy, alkynoyloxy, aryloyloxy, alkoxycarbonyloxy, aryloxycarbonyloxy or alkylsilyloxy;
E and F are independently hydrogen or lower alkanoyloxy, alkenoyloxy, alkynoyloxy, aryloyloxy; or E and F together form an oxo;
G is hydrogen or hydroxy or lower alkanoyloxy, alkenoyloxy, alkynoyloxy, or aryloyloxy or;
G and M together form; an oxo or methylene or oxirane ring or M and F together form an oxetane ring;
J is hydrogen or hydroxy or lower alkanoyloxy, alkenoyloxy, alkynoyloxy, or aryloyloxy or
I is hydrogen or hydroxy or lower alkanoyloxy, alkenoyloxy, alkynoyloxy, or aryloyloxy; or
I and J together form an oxo; and
K is hydrogen or hydroxy, lower alkoxy, alkanoyloxy, alkenoyloxy, alkynoyloxy, or aryloyloxy
P and Q are independently hydrogen or hydroxy or lower alkanoyloxy, alkenoyloxy, alkynoyloxy, aryloyloxy or alkoxycarbonyloxy, aryloxycarbonyloxy or alkylsilyloxy
P and Q together form an oxo; and
S and T are independently hydroxy, alkoxy, aryloxy, or lower alkanoyloxy, alkenoyloxy, alkynoyloxy, or aryloyloxy; and
U and V are independently hydrogen or lower alkyl, alkenyl, alkynyl, aryl or substituted aryl or heteroaryl and
W is hydrogen, alkanoyl, alkenoyl, alkynoyl, aryloyl, heteroaryloyl, alkoxycarbonyl, aryloxycarbonyl, heteroaryloxycarbonyl.
The taxol alkyl groups either alone or with the variable substituents defined above are preferably lower alkyl containing from one to six carbon atoms in the principal chain and up to 10 carbon atoms. They may be straight or branched chain such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, hexyl and the like. The taxol alkenyl groups either alone or with the various substituents defined above are preferably lower alkenyl containing from two to six carbon atoms in the principal chain and up to 10 carbon atoms. They may be straight or, branched chain and include ethenyl, propenyl, isopropenyl, butenyl, isobutenyl, pentenyl hexenyl, and the like.
The taxol alkynyl groups, either along or with the various substituents defined above are preferably lower alkynyl containing from two to six carbon atoms in the principal chain and up to 10 carbon atoms. They may be straight chain and include ethynyl, propynyl, butynyl, isobutynyl, pentynyl hexynyl and the like.
Exemplary alkanoyloxy include acetate, propionate, butyrate, valarate, isobutyrate and the like, the more preferred alkanoyloxy is acetate.
The taxol aryl moieties either alone or with various substituents contain from 6 to 10 carbon atoms and include phenyl, -naphthyl etc. Substituents include alkanoxy, hydroxy, halogen, alkyl, aryl, alkenyl, acyl, acyloxy, nitro, amino; amido etc. phenyl is the more preferred aryl.
As defined herein, the term aryloyloxy includes aromatic heterocyclic moieties, the term aryl includes any compound having an aromatic ring of which no hetero atom is a member, and the term heteroaryl includes any compound having an aromatic ring which comprises a hetero atom.
Preferred values of the substituents A, B, D, L, E, F, G, M, I, J, K, P, Q, S, T, U, V, W, X and Y are enumerated below in Table-1
According to this invention, there is provided a process for the preparation of 3(alk-2-ynyloxy)carbonyl-5-oxazolidine carboxylic acid. The process is illustrated in reaction 1: 
In this process, isoserine 12 is the starting material. In the preferred starting material, R is aryl and the most preferred R is phenyl. The arylisoserine has two asymmetric carbons, all optically active forms such as enantiomers, diastereomers, racemic mixtures and other mixtures are contemplated here. The preferred optically active form of pheneylisoserine is 2R, 3S. This phenylisoserine is commercially available.
Thus, phenylisoserine is condensed with alk-2-ynyl haloformate in presence of some base such as alkali hydroxide or carbonate or bicarbonate or any other acid neutralising chemical. Herein, alk-2-ynyl groups, either alone or with the various substitutents defined above are preferably lower alk-2-ynyl containing from three to six carbon atoms in the principal chain and up to 10 carbon atoms and include but-2-ynyl, pent-2-ynyl, prop-2-ynyl, hex-2-ynyl and the like.
The most preferred haloformate is prop-2-ynyl chloroformate which is available commercially. This can be prepared from diphosgene/triphosgene and propargyl alcohol following the procedure given in Helv.Chim.Acta. Vol 77, 561, 1994.
The most preferred base here is sodium bicarbonate. The condensation is carried out by addition of chloroformate into a solution of isoserine in aqueous bicarbonate over a period of 5-30 min, most preferably over 10-15 min. The acidification of the reaction mixture affords the N-(alk-2-ynyloxy)carbonylisoserine 13. The most preferred one is where R1 is phenyl, R2, R3, and R4 are hydrogen.
N-(Alk-2-ynyloxy)carbonylisoserine is then converted into corresponding ester in presence of an alcohol and an activating agent such as dicyclohexylcarbodiimide or carbonyldiimidazole. The most preferred alcohol is methanol and the most preferred activating agents is carbonyldiimidazole. The esterification can be carried out in various solvents such as dichloromethane, chloroform, ethyl acetate, tetrahydrofuran, acetonitrile or dimethyl-formamide. The most preferred solvent is acetonitrile. The esterification is carried out by first mixing acid with activating agent for 0.5-6 hr, most preferably for 1 hr, at 0-25xc2x0 C., most preferably at 10xc2x0 C. Then the alcohol is introduced in the mixture over a period of 10-60 min, most preferably over 30 min and the mixing is done for 1-6 hr, most preferably for 2 hr.
Methyl ester is alternatively prepared by condensing isoserine with diazomethane. The ethereal solution of diazomethane is prepared from following the standard procedure described in Vogel""s Text book of Practical Organic Chemistry, 1989, page 432. The methylation is effected by mixing the solution of N-(alk-2-ynyloxy)carbonylisoserine in tetrahydrofuran with excess ethereal solution of diazomethane. The resultant mixture is stored for 1-6 hr, most preferably for 2 hr at 5-25xc2x0 C., most preferably at 10xc2x0 C., for complete reaction.
In the most preferred ester 14, R1 is phenyl, R2 R3 and R4 are hydrogen; and R5 is methyl.
N-(alk-2-ynyloxy)carbonylisoserine ester 14 is converted into oxazolidine 15 in presence of chemical such as alkoxyalkene or gem-dialkoxyalkane or 1,1,1-trialkoxyalkane. These chemicals are basically reagents for protection of vicinal functional groups such as diols or amino/amidoalchohols in cyclic form. The appropriate chemical is chosen on the basis of type of protecting group required. The most preferred alkoxylalkene, 2-methoxypropene, is available commercially. The reactions are catalysed by p-arylsulfonic acid or their pyridinium salt. The most preferred catalyst is pyridinium p-toluenesulfonate. The protection with 2-methoxypropene results in formation of 3-(alk-2-ynyloxy)carbonyl-5-oxazolidine carboxylic ester 15. In the most preferred oxazolidine ester, R1, R2 and R3 are hydrogen, R4 is phenyl; R5, R6 and R7 are methyl.
3-(Alk-2-ynyloxy)carbonyl-5-oxazolidine carboxylic ester is finally converted into corresponding acid, by hydrolysis with alkali hydroxide or carbonate and mineral acid used successively. The hydrolysis is effected by mixing of a solution of ester in alcohol with an Aq. alkali hydroxide. The most preferred alcohol is methanol and the most preferred alkali hydroxide is sodium hydroxide. The mixing is done for 1 to 6 hr, the most preferably for 3 hr. After the hydrolysis is over, the reaction mixture is acidified to pH 3-6, most preferably 4.5. The acid 16 is isolated by extraction with dichloromethane. In the most preferred acid, R1, R2, and R3 are hydrogen, R4 is phenyl; R5 and R6 are methyl.
There is also provided a process of preparation of taxol and synthetic taxol analogs from 3-(alk-2-ynyloxy)carbonyl-5-oxazolidine carboxylic acids and taxane alcohols, which can be derived from natural or unnatural sources having the following reaction as an example: 
The most preferred 3-(alk-2-ynyloxy)carbonyl-5-oxazolidine carboxylic is 3-(prop-2-ynyloxy)carbonyl-2,2-dimethyl-4-phenyl-5-oxazolidine carboxylic acid. The preferred tetracyclic taxane alcohols are 7-O-triethylsilylbaccatin, 7-O-(2,2,2-trichloro-ethoxy)carbonylbaccatin, 7,10-di-O-(2,2,2-trichloroethoxy)carbonyl-10-deacetylbaccatin, 7, 10-di-O-benzyloxycarbonyl-10-deacetylbaccatin, 7-O-t-butyloxycarbonylbaccatin, 7,10-di-O-t-butyloxycarbonyl-10-deacetylbaccatin, 7-O-(prop-2-ynyloxy)carbonylbaccatin, and 7,10-di-O-(prop2-ynyloxy)carbonyl-10-deacetyl baccatin. The most preferred alcohols are 7-O-(2,2,2-trichloroethoxy)carbonylbaccatin, 7,10-di-O-( 2,2,2-trichloroethoxy)carbonyl-10-deacetylbaccatin.
7-O-(2,2,2-trichloroethoxy)carbonylbaccatin, 7,10-di-O-(2,2,2-trichloroethoxy)carbonyl-10-deacetylbaccatin are synthesized from baccatin and 10-deacetylbacctin respectively in presence of 2,2,2-trichloroethoxy chloroformate and organic base. The preferred bases are pyridine, 4-dimethylaminopyridine or imidazole or like. The most preferred base is pyridine.
The 3-(prop-2-ynyloxy)carbonyl-2, 2-dimethyl-4-phenyl-5 oxazolidine carboxylic acid 16 is converted into taxane ester 18 in presence of taxane alcohol such as 7-O-(2,2,2-trichloroethoxy)carbonylbaccatin 17 and condensing agent, preferably dicyclohexylcarbodiimde and organic base preferably 4-dimethylaminopyridine. This process can also be used to prepare various taxane esters 10 contemplated within the present invention from various 3-(alk-2-ynyloxy)carbonyl-5oxazolidine carboxylic acids and taxane alcohols.
The opening of the oxazolidine ring in ester 18 is effected by removal of prop-2-ynyloxy carbonyl with ammonium tetrathiomolybadate preferably benzyltriethyl ammonium tetrathiomolybadate which is prepared according to the known procedure described in Syn.Comm., 22(22), 3277-3284(1992). The removal of prop-2-ynyloxycarbonyl group is effected by exposing the mixture of ester and acetonitrile to ultrasound for 1-6 hr, most preferably for 3 hr. The reaction mixture is diluted with dichloromethane and then washed with water. Removal of the solvents gives the intermediate 19, free amine. The same procedure can be applied to prepare other free amino taxane intermediates 11 (where W is hydrogen) contemplated within the present invention.
The resultant free amine can then be converted into taxol 1 or non-natural taxols such as 2 and 11 contemplated within the present invention following the known methods as illustrated in scheme-3 
The invention will now be explained in greater details with the help of the accompanying examples: